Coking tower life improvement

8/13/2019 Coking tower life improvement

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ANALYSIS OF COKE DRUM CRACKING FAILURE MECHANISMS &COMMENTS ON SOME PUBLISHED RESULTS

Coke Drum Reliability Workshop

Calgary, Alberta

17 Sep 2009

J Aumuller, P. Eng.

Z Xia, Ph. D., P. Eng.

COKING.COM 2009COKER DRUM CRACKING

EDA

Engineering Design & Analysis Ltd.


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Coke Drums

Coke drums are large pressure vesselsused in oil sands plants & refineriesfor the recovery of hydrocarbonproduct from reduced bitumen

30 feet x 90 feet height

operate to 50 psig, 900 F, cyclic

Construction materials

composite plate construction, 1

nominal thickness consisting of TP 410S stainless steel cladding

carbon steel or low alloy carbonsteel (CS, C - Mo, Cr - Mo)

Problem cracking of shell, attributedto presence of bulges and low cyclefatigue



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Coke Drum Bulging



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Why stress determination

vessel bulging and cracking attributable to mechanicalmechanism rather than metallurgical

primary mechanical failure mechanism is

low cycle thermal strain cycling

What are

the various loadings their nature

contribution to the proposed failure mechanism



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0

100

200

300

400

500

600

700

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0

time in [hours]

CS 4 CS 5

oil in

waterquench

coke out

pressure


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Shell OD Strain - Measured

-200

0

200

400

600

800

1000

1200

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0

time in [hours]

str

ainin[ue]

CS 4 CS 5

Steam test

Vapor

heat

Oil in

Water quench


NB - the measured strainsare not necessarily damaging


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Coke Drum Vasing, Hot, Cold Spots, & Transients

vasing action is a nominal response

bitumen filling, water filling occur over

same repeating nominal time period,nominal temperature range

plug flow nature

drum vasing also occurs

during coke cool-down due to insulatingeffect as coke forms, liquid solid

water quench addition

localized distortions superimposed

system hydraulics cause channel flow& deviations in temperature strain,stress

Steam / Bitumen /Water

Diameter decreasedue to water quenchtemperature

Drum diameter

decrease lagsdecrease in lowerelevations



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Comments on available published data

Field data validity

temperature data likely okay, except where insulation is left off

strain data is highly suspect fundamental errors in methodology

thermal strain, eTH is

inconsistently accounted for, or

not accounted for entirely

evaluation of strain gauge readings is incorrect

closed form expressions are not appropriate, equivalent strainexpression premised on 2D model; however, 3D strain state ispresent

no data measured at most susceptible locations



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Comments on available published data

base material failure is accelerated likely due to HEAC

field & published data regarding base material failure

proceeds rapidly in comparison to clad layer failure,

months versus years

dependant on operational specifics



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Temperature loading understanding the fundamentals

for isotropic material, temperature increase results

in uniform strain

no stress when body is free to deform

the total strain in a body, eT is composed of two components

mechanical portion = eM [due to pressure, weight, + others]

thermal portion = eTH

then, eT = eM + eTH when thermal growth is constrained, eT = 0 eM = - eTH since eTH = T, where coefficient of thermal

expansion or CTE and, the coke drum is in a biaxial

stress state, then

thermal stress, TH = - E T / (1 )



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Temperature loading [contd]

thermal expansion in coke drum is constrained due to severalmechanisms

skirt structure

cladding base material differential expansion due tomismatch in coefficient of thermal expansion, CTE

T between adjacent parts of the structure due to varyingexposure to incoming streams, i.e. bitumen [hot] and quenchwater [cold]

100 F 800 F

[in/in/F] [in/in/F]

CTE-clad 6.0E-6 7.1E-6

CTE-base 6.6E-6 8.9E-6



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Thermal Expansion vs Temperature for Various

Materials of Construction

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 100 200 300 400 500 600 700 800 900 1000

Temperature [F]

CTE

[10^-6

/F]

C 1/2Mo

1 1/4 Cr

2-1/4 Cr

410S

N06625


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E (Young's Modulus) vs Temperature

15.0

17.5

20.0

22.5

25.0

27.5

30.0

32.5

35.0

0 100 200 300 400 500 600 700 800 900 1000

Temperature [F]

E

-[10^

6

psi]

C 1/2Mo

1 1/4 Cr

2-1/4 Cr

410S

N06625


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Temperature loading [contd] - Temperature - Stress Profile Comparisons



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Nature of Drum Failures

Low Cycle Fatigue da / dN

characterized by high strain low cycle

exacerbated by presence of code acceptable defects

cladding crack failure initiation < 1,000 ~ 2,000 cycles

cladding crack propagation thru thickness ~ 2,500 cycles

Environmentally assisted fatigue da / dt exposure of base material to hydrogen assisted mechanism

short time to through failure hours to months

cleavage surfaces evident



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Number of Drums Reporting 1st Through Wall Crack API Survey


* Final Report, 1996 API Coke Drum Survey, Nov 2003, API, Washington, D.C.

0

10

20

30

40

50

60

70

80


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Nature of Drum Failures contd

Upper bound strain

measured strain range, = 2,500 ue ~ 3,400 ue

calculated possible, = 5,140 ue ~ 14,400 ue

Micro-strain-

ue

Time

measurements fall wellbelow values governedby system parameters

system parameters

indicate that strainsrepeat and will causefailure at susceptiblelocations



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2,570 3,400 5,140 7,200 14,400

N 100,000 25,000 4,800 2,500 900

Years 274 68 13 7 2.5

- N Low Cycle Strain Life Curve for SA 387 12 Plate [2 Cr 1Mo]

* Sonoya, K., et al., ISIJ International v 31 (1991) n 12 p 1424 - 1430

extremes

failure can occur within 2.5 years

potential service life of 274 years

actual performance of unit is

function of system specifics



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2,570 3,400 5,140 7,200 14,400

77.1 102.0 154.2 216.0 432.0

N 10,000 4,200 1,200 550 70

Years 27 11.5 3 1.5 0.2

- N Low Cycle Strain Life Curve per ASME VIII Div 2


ASME VIII Div 2 S N chart is notappropriate for service life determination


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Influence of Internal Defects

Code allows internal defects

For material thickness over inch to 2 inch, inclusive [19 mm to 50.8 mm]

Maximum size for isolated indication is [6.4 mm] diameter

Table limiting defect size is given in ASME VIII Div 1



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-60,000

-50,000

-40,000

-30,000

-20,000

-10,000

0

10,000

20,000

30,000

40,000

50,000

60,000

0 5,000 10,000 15,000 20,000 25,000 30,000

Time in [sec]

Stressin

[psi]

ID SURF ID DEFECT OD SURF

Stress at Internal Defects

Stress at clad

Stress at internal defect

largest strains/stresses at

clad

internal defects

local distortions

maximum range of strains& stresses known due to

system parameters


Stress at OD surface


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Conclusions

field measurement techniques problematic

thermal strain interpreted as mechanical strain

measured strains well below upper bound strains

strains at internal defects inaccessible, no measurement

strains at material interface inaccessible, no measurement

upper bound approach determines maximum strains obtainable

strain level, # of exposure incidents governed by system hydraulics

strain level, # of exposures govern service life

local shell deformations will further affect strain levels

crack initiation function of clad & base material integrity

through-wall base material failure related to HEAC susceptibility



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Evaluation

improve field measurement techniques

improve design procedures

ASME VIII Div 1 not adequate to address complex loadings

more detailed & accurate estimation of stress required

need to consider more than material yield strength properties

material selection opportunities less expensive options forsame performance

preventative maintenance & repair opportunities identifiable



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Follow up work opportunities

develop improved field stress measurement technique

detection of internal defects and assessment technique

assessment of influence of local shell distortions

material constitutive modeling for better FEA modeling

characterization of base material performance in HEACenvironment

identify alternative clad materials

develop appropriate design methodologies for coke drum

Joint industry program to leverage industry & NSERC resources



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Contact

Dr. Zihui Xia, University of Alberta

[email protected]

T: 780 492 3870

John Aumuller, EDA Ltd.

[email protected] T: 780 484 5021


EDAEngineering Design & Analysis Ltd.

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